Engines may be configured with an exhaust heat recovery system for recovering heat from exhaust generated in an internal combustion engine. The heat recovered by an exhaust heat exchanger may be converted to electrical energy and stored in a system battery. Electrical energy from the battery may be opportunistically utilized for functions such as operating a motor of a compressor, operating a pump, cylinder head heating, vehicle cabin heating and lighting, etc., thereby improving engine and fuel efficiency.
Various approaches are provided for exhaust heat recovery. In one example, as shown in US 20130219872, Gibble et al. discloses a heat recovery and thermal management system including a thermoelectric device used for recovering heat from exhaust gas and converting the heat to electrical energy. A bottoming cycle, such as a Rankine cycle, is used in the heat recovery system for electricity generation. The electrical energy produced from the exhaust heat is stored in a battery and later used for functions such as vehicle cabin heating.
However, the inventors herein have recognized potential disadvantages with the above approach. As one example, for efficient operation of a bottoming cycle, such a Rankine cycle, a steady supply of exhaust thermal energy that is within a target energy range is required to maintain a higher than threshold pressure ratio at the expander of the bottoming cycle. The exhaust thermal energy may be determined as a function of the exhaust temperature and the exhaust flow-rate. In Gibble et al., during conditions when there is a lower than target supply of exhaust thermal energy, the bottoming cycle efficiency may decrease. During conditions when there is a higher than target flux of exhaust heat, heat recovery may be limited by the size of the bottoming cycle components. In particular, in order to recover the higher levels of heat, larger bottoming cycle components, such as a larger expander, a larger compressor, and/or a larger (or more powerful) pump may have to be used. However, such larger components may have a higher thermal inertia, causing energy losses, and may also increase component cost along with packaging concerns. If thermoelectric components are used for exhaust heat recovery, higher than target exhaust thermal energy may cause damage to the thermally sensitive components. In order to reduce fluctuations in exhaust thermal energy from reaching the bottoming cycle components, a bypass passage of the heat exchanger may be used to route exhaust during conditions when the exhaust thermal energy is outside the target range. However, when exhaust bypasses the bottoming cycle components, exhaust heat available for recovery decreases.
The inventors herein have identified an approach by which the issues described above may be at least partly addressed. One example method comprises: when exhaust thermal energy is higher than a first threshold, flowing exhaust through a heat exchanger after storing a portion of the thermal energy at a thermal energy storage device upstream of the heat exchanger; and when exhaust thermal energy is lower than a second threshold, flowing exhaust through the heat exchanger after drawing thermal energy from the thermal energy storage device. In this way, by opportunistically storing energy from exhaust in a thermal storage device and subsequently using that energy to maintain a steady supply of exhaust thermal energy at the heat exchanger, exhaust heat recovery efficiency may be increased.
In one example, the engine exhaust system may be configured with a post-catalyst thermal storage device and a heat exchanger coupled to an exhaust passage leading to a tailpipe. The heat exchanger may be part of a bottoming cycle, the bottoming cycle further comprising an expander (such as a turbine), a condenser, and a pump. A bypass passage may be coupled to the exhaust passage across the thermal storage device, enabling post-catalyst exhaust to be routed to the heat exchanger bypassing the thermal storage device. A recirculation passage may be coupled to the exhaust passage from downstream of the heat exchanger to upstream of the thermal storage device. Routing of exhaust to the heat exchanger may be adjusted based on the exhaust thermal energy, estimated as a function of the exhaust temperature and exhaust flow-rate, so that the thermal energy reaching the heat exchanger can be maintained within a target energy range. During conditions when the exhaust thermal energy is within the target range, exhaust may be directly routed to the heat exchanger via the bypass passage. Exhaust heat recovered at the heat exchanger may be converted to electrical energy at the bottoming cycle, and the electrical energy may be stored in the battery for later use. Also during engine cold-start conditions, the entire volume of exhaust may be directly routed through the heat exchanger to expedite cold-start exhaust heat recovery for engine heating. During conditions when the exhaust thermal energy is higher than the target range, a first portion of exhaust corresponding to the excess thermal energy may be routed through the thermal storage device, and the excess thermal energy may be stored at the thermal storage device. A remaining second portion of exhaust, corresponding to the target range thermal energy, may be routed to the heat exchanger via the bypass passage. Alternatively, the entire volume of exhaust may be routed via the thermal storage device wherein exhaust heat may be stored until its storage capacity is reached. During conditions when the exhaust thermal energy is lower than the target range (such as when the exhaust temperature is low or when the exhaust flow rate is low), a portion of cooled exhaust may be recirculated from downstream of the heat exchanger to upstream of the thermal storage device via the recirculation passage. The portion of exhaust may then be heated by withdrawing thermal energy inform the thermal storage device, the heated exhaust then combining with fresh exhaust before flowing through the heat exchanger. During conditions when the exhaust thermal energy is lower than the target range due to low exhaust flow rates, in addition to the recirculation of exhaust, ambient air may be drawn into the recirculation passage using a blower, and the air-exhaust mixture may be heated by withdrawing heat from the thermal storage device before flowing through the heat exchanger.
In this way, by selectively adjusting exhaust flow through an exhaust heat exchanger, a steady supply of exhaust thermal energy that is within a target energy range may be provided to the heat exchanger. The technical effect of maintaining a steady supply of exhaust thermal energy is that a target pressure ratio may be maintained at the expander of the bottoming cycle, thereby maintaining the efficiency of the bottoming cycle. The technical effect of opportunistically storing excess exhaust energy in a thermal storage device is that during low exhaust thermal energy conditions, thermal energy previously stored in the thermal storage device may be used to heat a part of the exhaust entering the heat exchanger, thereby raising the exhaust thermal energy to the target range, and enabling the steady flow of exhaust energy to be maintained at the heat exchanger and the bottoming cycle. By drawing in ambient air during low exhaust flow rate conditions, and heating the ambient air-exhaust mixture using energy stored at the thermal storage device, the exhaust flow-rate through the heat exchanger may be raised and the thermal energy of the exhaust reaching the heat exchanger may be maintained at the desired level. By maintaining the exhaust thermal energy reaching the heat exchanger within a target range by opportunistically storing and withdrawing energy at/from a thermal storage device, the bottoming cycle coupled to the heat exchanger may be efficiently operated for electrical energy generation over a wider range of vehicle operating conditions, including during engine-off periods in hybrid electric vehicles. In addition, a higher efficiency may be achieved while relying on smaller and lighter components. By storing exhaust heat as electrical energy in a battery, exhaust heat that would have otherwise been wasted may be effectively used for operating pumps, providing heat to vehicle components, operating motors, etc. Overall, by enhancing exhaust heat recovery, engine performance and fuel efficiency are improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.